Question

Question 4

Write a regular expression that generates each of the following language constructs:

(1) String constants with the following specifications:

A string constant consists of any sequence of characters enclosed by the quotation marks: “ and ”

      The sequence may be empty.             

      The sequence cannot span multiple lines.

      Don’t worry about escape characters (assume that they won’t appear in the input).

     

(2)  Multiple-line comment in C, C++ and JAVA with the following specifications:

       The comment consists of any sequence of characters enclosed by the delimiters /* and */.

        The sequence may be empty.

        The sequence cannot have the * character.

        Don’t worry about escape characters (assume that they won’t appear in the input).

(3) An identifierwith the following specifications:                                                                                                                       

The identifier consists of alphabetic characters [a-Z], numbers [0-9] and underscores.

The first symbol must be an alphabetic character.

The last symbol must be an alphabetic character or an underscore.

The identifier must have at least one number.

Answer 1

1.
Answer:
Let Σ denotes set of all valid alphabets in the language then Σ* denote set of string all possible string over Σ,

then the regular expression for String constant consists of any sequence of characters enclosed by

the quotation marks: " and " is given by ="Σ*" (the required regular expression)
2.
Answer:
Let Σ denotes set of all valid alphabets in the language, Since * can't in the string in comments

we construct Σ1=Σ^*-{*}then

The comment consisting of any sequence of characters is :/*{Σ1}^**/ (the required regular expression)

3.
Answer:
let alpabets=[a-Z]={a,b,c....X,Y,Z}
   numbers=[0-9] ={0,1,2,...9}
   identifier=alpabets(alpabets + numbers + {_})^*numbers(alpabets + numbers + {_})^*(alpabets + {_})